But the Nineties have not started so well for them - at least on British television. Their latest ITV comedy drama series, Full Stretch, set in a luxury car-hire company, has not picked up nearly as many passengers as they hoped.
The first episode, on 5 January, was watched by 7.7 million people, compared with 11.98 million for Spender on BBC 1 at the same time (which, ironically, Mr La Frenais helped to devise). Episode two attracted just 6.8 million, not much more than the spoof detective series Police Squad, repeated on BBC 2. By week three the audience had climbed back to 7 million, but this was still far short of the 10 million that Carlton TV's managing director, Paul Jackson, says peak-time ITV shows must reach.
It is the critical reception of Full Stretch, however, that has most stung its authors. 'It gets my Dimmest Newcomer award and makes the Darling Buds of May look cerebral,' the Daily Telegraph's reviewer fulminated. 'I'm astonished by the meanness and negativity of the reaction,' Mr La Frenais says. 'In all our years, we've never been so surprised by the attitude of Fleet Street.'
'There was a real arms-folded, determinedly unimpressed, sniping attitude to all the programmes coming from the new franchises,' Mr Clement says. 'I think we've suffered from that.'
In fact, the two writers have much to gain from ITV's new order. They are shareholders in SelecTV, the thrusting independent production group that owns 15 per cent of Meridian, the new South of England franchise holder for whom Full Stretch was made.
None the less, they are anxious about the future of the series. As long-time residents of Los Angeles who refuse even to watch American television let alone write for it, they know from bitter experience how creativity and originality can be stifled in a system geared to commercialism.
'Over here, you can still do a quality show and give it a chance, nurture it along,' Mr La Frenais says. 'But things are changing here, too. In the past we wouldn't have been at all concerned about the reaction to Full Stretch, because we know it's a good show that needs another series to develop. But in the climate at the moment, we're nervous that it won't be allowed that chance.'
'In America, they used to let shows build - such as All in the Family, the US version of Till Death Us Do Part, which wasn't a hit in its first season, but they stuck with it,' Mr Clement says. 'Subsequently, they haven't nurtured anything; programmes have to be an immediate hit, or they're taken off. I think there's the danger of that happening in Britain. Our partner, Allan McKeowan, is taping a show in America at the moment. He went into a meeting about it yesterday, and there were 25 people in the room giving him script notes.
'That's absolute insanity, and it's the reason we won't write for American television. They make a few good programmes, but it's an uphill struggle and it drives you crazy. Over here, it's still fun. I hope it stays that way.
Mr Clement and Mr La Frenais, both aged 54, understand each other so well that they frequently finish each other's sentences, but they look very different: Mr Clement is a tall, blond, urbane southerner from Westcliff-on-Sea; Mr La Frenais is small, dark and hirsute, with a rather wild look in his eye and strong traces of a native Newcastle accent. They still wear the longish hair of the Sixties and the air of ageing swingers, although they enjoy long-lasting marriages to American wives.
They were introduced by a friend in the Uxbridge Arms in Notting Hill Gate in 1962. Mr Clement, a trainee in the BBC's African Service, and Mr La Frenais 'a trainee in nothing', then wrote a sketch for Mr Clement to use as an exam piece on his director's course. Soon they were summoned by Michael Peacock, the original controller of BBC 2. 'Do you think there's a series in this?' he asked. 'Oh, yes,' they replied, with an assurance they were far from feeling.
That is how they came to write The Likely Lads. 'It was incredibly exciting even just being in the BBC canteen,' said Mr Clement. 'First, there was the BBC old guard, then there were all these badly dressed satirists who had been to university and didn't look like Sixties people at all. On the other side of the room was Top of the Pops, and they did look like Sixties people.
'The intellectuals expected us to entertain them because we were in light entertainment. But in fact we were doing something that wasn't like LE at all. We were trying to write what we'd seen not on television but in movies - those black and white Sixties movies of the grainy North. We felt we should have been in drama.'
In 1975, they moved to Los Angeles on the strength of the Porridge movie and their West End stage success, Billy. But after an ill-fated attempt to make an American version of Porridge, they decided to write for television only in Britain. With Allan McKeowan, they set up one of the UK's first independent companies, Witzend Productions, to make comedy drama series such as Auf Wiedersehn, Pet and Shine on Harvey Moon for ITV.
Life in California suits them. 'It's nice living in one place and working in another,' Mr La Frenais says. 'It stops us getting dull.' Much of their effort has been concentrated on becoming successful screenwriters: their script for Alan Parker's Dublin comedy The Commitments was an unexpected hit in Hollywood, and two other film scripts are to go into production in Hollywood later this year. By then, yet another new Clement/La Frenais comedy drama series, Over the Rainbow, featuring two of the young stars of The Commitments, will have been completed by SelecTV for Meridian.
What keeps them working so hard after 30 years? 'The pressure is the same as it's always been,' Mr Clement says. 'We want to do excellent work, and we don't want those buggers in Fleet Street to say we've lost it.'